This is a patch to fix pathfinding issues on SMP 1.6.2 associated with zombies targetting unreachable villagers and totally killing server performance.
This solution was taken from Markku's comment here: https://mojang.atlassian.net/browse/MC-17630?focusedCommentId=92590&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-92590